From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Andrew Rybchenko" <andrew.rybchenko@oktetlabs.ru>,
"Olivier Matz" <olivier.matz@6wind.com>
Cc: <dev@dpdk.org>, "Beilei Xing" <beilei.xing@intel.com>,
"Bruce Richardson" <bruce.richardson@intel.com>,
"Jerin Jacob Kollanukkaran" <jerinj@marvell.com>
Subject: RE: [PATCH v3] mempool: fix get objects from mempool with cache
Date: Tue, 4 Oct 2022 16:42:03 +0200 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35D87389@smartserver.smartshare.dk> (raw)
In-Reply-To: <20221004125354.3438386-1-andrew.rybchenko@oktetlabs.ru>
> From: Andrew Rybchenko [mailto:andrew.rybchenko@oktetlabs.ru]
> Sent: Tuesday, 4 October 2022 14.54
> To: Olivier Matz
> Cc: dev@dpdk.org; Morten Brørup; Beilei Xing; Bruce Richardson; Jerin
> Jacob Kollanukkaran
> Subject: [PATCH v3] mempool: fix get objects from mempool with cache
>
> From: Morten Brørup <mb@smartsharesystems.com>
>
> A flush threshold for the mempool cache was introduced in DPDK version
> 1.3, but rte_mempool_do_generic_get() was not completely updated back
> then, and some inefficiencies were introduced.
>
> Fix the following in rte_mempool_do_generic_get():
>
> 1. The code that initially screens the cache request was not updated
> with the change in DPDK version 1.3.
> The initial screening compared the request length to the cache size,
> which was correct before, but became irrelevant with the introduction
> of
> the flush threshold. E.g. the cache can hold up to flushthresh objects,
> which is more than its size, so some requests were not served from the
> cache, even though they could be.
> The initial screening has now been corrected to match the initial
> screening in rte_mempool_do_generic_put(), which verifies that a cache
> is present, and that the length of the request does not overflow the
> memory allocated for the cache.
>
> This bug caused a major performance degradation in scenarios where the
> application burst length is the same as the cache size. In such cases,
> the objects were not ever fetched from the mempool cache, regardless if
> they could have been.
> This scenario occurs e.g. if an application has configured a mempool
> with a size matching the application's burst size.
>
> 2. The function is a helper for rte_mempool_generic_get(), so it must
> behave according to the description of that function.
> Specifically, objects must first be returned from the cache,
> subsequently from the ring.
> After the change in DPDK version 1.3, this was not the behavior when
> the request was partially satisfied from the cache; instead, the
> objects
> from the ring were returned ahead of the objects from the cache.
> This bug degraded application performance on CPUs with a small L1
> cache,
> which benefit from having the hot objects first in the returned array.
> (This is probably also the reason why the function returns the objects
> in reverse order, which it still does.)
> Now, all code paths first return objects from the cache, subsequently
> from the ring.
>
> The function was not behaving as described (by the function using it)
> and expected by applications using it. This in itself is also a bug.
>
> 3. If the cache could not be backfilled, the function would attempt
> to get all the requested objects from the ring (instead of only the
> number of requested objects minus the objects available in the ring),
> and the function would fail if that failed.
> Now, the first part of the request is always satisfied from the cache,
> and if the subsequent backfilling of the cache from the ring fails,
> only
> the remaining requested objects are retrieved from the ring.
>
> The function would fail despite there are enough objects in the cache
> plus the common pool.
>
> 4. The code flow for satisfying the request from the cache was slightly
> inefficient:
> The likely code path where the objects are simply served from the cache
> was treated as unlikely. Now it is treated as likely.
>
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> ---
> v3 changes (Andrew Rybchenko)
> - Always get first objects from the cache even if request is bigger
> than cache size. Remove one corresponding condition from the path
> when request is fully served from cache.
> - Simplify code to avoid duplication:
> - Get objects directly from backend in single place only.
> - Share code which gets from the cache first regardless if
> everythihg is obtained from the cache or just the first part.
> - Rollback cache length in unlikely failure branch to avoid cache
> vs NULL check in success branch.
>
> v2 changes
> - Do not modify description of return value. This belongs in a separate
> doc fix.
> - Elaborate even more on which bugs the modifications fix.
>
> lib/mempool/rte_mempool.h | 74 +++++++++++++++++++++++++--------------
> 1 file changed, 48 insertions(+), 26 deletions(-)
Thank you, Andrew.
I haven't compared the resulting assembler output (regarding performance), but I have carefully reviewed the resulting v3 source code for potential bugs in all code paths and for performance, and think it looks good.
The RTE_MIN() macro looks like it prefers the first parameter, so static branch prediction for len=RTE_MIN(remaining, cache->len) should be correct.
You could consider adding likely() around (cache != NULL) near the bottom of the function, so it matches the unlikely(cache == NULL) at the top of the function; mainly for symmetry in the source code, as I expect it to be the compiler default anyway.
Also, you could add "remaining" to the comment:
/* Get the objects directly from the ring. */
As is, or with suggested modifications...
Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
next prev parent reply other threads:[~2022-10-04 14:42 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-26 15:34 [RFC] mempool: rte_mempool_do_generic_get optimizations Morten Brørup
2022-01-06 12:23 ` [PATCH] mempool: optimize incomplete cache handling Morten Brørup
2022-01-06 16:55 ` Jerin Jacob
2022-01-07 8:46 ` Morten Brørup
2022-01-10 7:26 ` Jerin Jacob
2022-01-10 10:55 ` Morten Brørup
2022-01-14 16:36 ` [PATCH] mempool: fix get objects from mempool with cache Morten Brørup
2022-01-17 17:35 ` Bruce Richardson
2022-01-18 8:25 ` Morten Brørup
2022-01-18 9:07 ` Bruce Richardson
2022-01-24 15:38 ` Olivier Matz
2022-01-24 16:11 ` Olivier Matz
2022-01-28 10:22 ` Morten Brørup
2022-01-17 11:52 ` [PATCH] mempool: optimize put objects to " Morten Brørup
2022-01-19 14:52 ` [PATCH v2] mempool: fix " Morten Brørup
2022-01-19 15:03 ` [PATCH v3] " Morten Brørup
2022-01-24 15:39 ` Olivier Matz
2022-01-28 9:37 ` Morten Brørup
2022-02-02 8:14 ` [PATCH v2] mempool: fix get objects from " Morten Brørup
2022-06-15 21:18 ` Morten Brørup
2022-09-29 10:52 ` Morten Brørup
2022-10-04 12:57 ` Andrew Rybchenko
2022-10-04 15:13 ` Morten Brørup
2022-10-04 15:58 ` Andrew Rybchenko
2022-10-04 18:09 ` Morten Brørup
2022-10-06 13:43 ` Aaron Conole
2022-10-04 16:03 ` Morten Brørup
2022-10-04 16:36 ` Morten Brørup
2022-10-04 16:39 ` Morten Brørup
2022-02-02 10:33 ` [PATCH v4] mempool: fix mempool cache flushing algorithm Morten Brørup
2022-04-07 9:04 ` Morten Brørup
2022-04-07 9:14 ` Bruce Richardson
2022-04-07 9:26 ` Morten Brørup
2022-04-07 10:32 ` Bruce Richardson
2022-04-07 10:43 ` Bruce Richardson
2022-04-07 11:36 ` Morten Brørup
2022-10-04 20:01 ` Morten Brørup
2022-10-09 11:11 ` [PATCH 1/2] mempool: check driver enqueue result in one place Andrew Rybchenko
2022-10-09 11:11 ` [PATCH 2/2] mempool: avoid usage of term ring on put Andrew Rybchenko
2022-10-09 13:08 ` Morten Brørup
2022-10-09 13:14 ` Andrew Rybchenko
2022-10-09 13:01 ` [PATCH 1/2] mempool: check driver enqueue result in one place Morten Brørup
2022-10-09 13:19 ` [PATCH v4] mempool: fix mempool cache flushing algorithm Andrew Rybchenko
2022-10-04 12:53 ` [PATCH v3] mempool: fix get objects from mempool with cache Andrew Rybchenko
2022-10-04 14:42 ` Morten Brørup [this message]
2022-10-07 10:44 ` [PATCH v4] " Andrew Rybchenko
2022-10-08 20:56 ` Thomas Monjalon
2022-10-11 20:30 ` Copy-pasted code should be updated Morten Brørup
2022-10-11 21:47 ` Honnappa Nagarahalli
2022-10-30 8:44 ` Morten Brørup
2022-10-30 22:50 ` Honnappa Nagarahalli
2022-10-14 14:01 ` [PATCH v4] mempool: fix get objects from mempool with cache Olivier Matz
2022-10-09 13:37 ` [PATCH v6 0/4] mempool: fix mempool cache flushing algorithm Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 1/4] mempool: check driver enqueue result in one place Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 2/4] mempool: avoid usage of term ring on put Andrew Rybchenko
2022-10-09 13:37 ` [PATCH v6 3/4] mempool: fix cache flushing algorithm Andrew Rybchenko
2022-10-09 14:31 ` Morten Brørup
2022-10-09 14:51 ` Andrew Rybchenko
2022-10-09 15:08 ` Morten Brørup
2022-10-14 14:01 ` Olivier Matz
2022-10-14 15:57 ` Morten Brørup
2022-10-14 19:50 ` Olivier Matz
2022-10-15 6:57 ` Morten Brørup
2022-10-18 16:32 ` Jerin Jacob
2022-10-09 13:37 ` [PATCH v6 4/4] mempool: flush cache completely on overflow Andrew Rybchenko
2022-10-09 14:44 ` Morten Brørup
2022-10-14 14:01 ` Olivier Matz
2022-10-10 15:21 ` [PATCH v6 0/4] mempool: fix mempool cache flushing algorithm Thomas Monjalon
2022-10-11 19:26 ` Morten Brørup
2022-10-26 14:09 ` Thomas Monjalon
2022-10-26 14:26 ` Morten Brørup
2022-10-26 14:44 ` [PATCH] mempool: cache align mempool cache objects Morten Brørup
2022-10-26 19:44 ` Andrew Rybchenko
2022-10-27 8:34 ` Olivier Matz
2022-10-27 9:22 ` Morten Brørup
2022-10-27 11:42 ` Olivier Matz
2022-10-27 12:11 ` Morten Brørup
2022-10-27 15:20 ` Olivier Matz
2022-10-28 6:35 ` [PATCH v3 1/2] " Morten Brørup
2022-10-28 6:35 ` [PATCH v3 2/2] mempool: optimized debug statistics Morten Brørup
2022-10-28 6:41 ` [PATCH v4 1/2] mempool: cache align mempool cache objects Morten Brørup
2022-10-28 6:41 ` [PATCH v4 2/2] mempool: optimized debug statistics Morten Brørup
2022-10-30 9:09 ` Morten Brørup
2022-10-30 9:16 ` Thomas Monjalon
2022-10-30 9:17 ` [PATCH v4 1/2] mempool: cache align mempool cache objects Thomas Monjalon
2022-06-24 10:23 [PATCH] net/af_xdp: make compatible with libbpf v0.8.0 Ciara Loftus
2022-10-05 9:50 ` [PATCH v3 0/6] " Andrew Rybchenko
2022-10-05 9:50 ` [PATCH v3] mempool: fix get objects from mempool with cache Andrew Rybchenko
2022-10-05 9:56 ` Andrew Rybchenko
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